1. Field of the Invention
The present invention relates to a battery power supply system, and more particularly to an inexpensive battery power supply system which may quickly release a reverse charge phenomenon.
2. Related Background Art
Interest to a global environment has recently been very much enhanced and a great expectation has been paid to a solar cell which is a representative one of battery power supplies which are inexhaustible and clean energy sources. Today, the cost of a solar cell is less than 1000 Yen/W and it is 1/10 when compared with that of a few years ago. The electricity generated by the solar cell is more expensive than the existing commercial electric power as of 1994 but it is said that the cost of the solar sell will go down to 200 Yen/W in 2000, when the electricity can be generated by the solar cell with a comparable cost to that of the commercial power supply.
Since the solar cell is operative only during the sunshine, some backup means for the night and rainy day is required when it is to be used in a home. To this end, a system for using the power supply and the solar cell output in linkage through an inverter has been proposed and it has been put into practice.
One example of such linkage system is shown in FIG. 13. A DC power generated by a solar cell is converted to an AC power by a DC-AC inverter 2 and it is supplied to a load 5. In the absence of sum shine, the commercial power is supplied to the load 5 through a break device 3, which disconnects the solar generation system from the system line in response to a short circuit accident in the system or an accident of the commercial system. In most cases, it stops the DC-AC inverter 2.
However, in many cases, the power generated by the solar cell is in excess in daytime. The flow of the excess power to the power system is called a "reverse current". By the reverse current, the electricity generated by the solar cell can be used without waste. Such a reverse current type solar cell generation system would be an ideal manner of use of the solar cell. However, a problem of "reverse charge phenomenon" which is inherent to the reverse current system has not yet been solved.
The "reverse charge phenomenon" may occur when the quantity of generation by the solar cell generation system and the power consumption of a load connected thereto are substantially equal. In such a case, even if the electricity of the commercial power supply system is stopped or it stops by some cause, the solar cell generation system cannot detect the stop of the commercial power system and the solar cell generation system continues the independent operation. As a result, lines which are not to be active are charged and a risk occurs in a maintenance operation of the lines. Further, when the commercial power system is reclosed, the commercial power system and the solar cell generation system are not in synchronism and an excess current is generated and the reclosure may fail. When many solar cell generation systems are used in future, the "reverse charge phenomenon" may occur over a wide area.
In order to prevent the "reverse charge phenomenon", various method have been proposed as shown in Table 1 and tests of those methods have been conducted but no definite solution therefor has been found yet.
TABLE 1 __________________________________________________________________________ Method Content Remarks __________________________________________________________________________ 1. Countermeasure on solar cell generation system (1) Methodic countermeasure a. Adoption of current .cndot.Use constant current source and .cndot.Can localize conditions of controlled inverter no function to maintain voltage. occurence of reverse charge operation. (2) Passive countermeasure a. Phase monitor system .cndot.Detect transitional variation of line volatge phase .cndot.Not operable in a complete balanced condition. .cndot.Detect abrupt change in phase difference Very effective for other cases. inverter output voltage and current. b. Harmonics monitor system .cndot.Monitor harmonic voltage generated by line .cndot.Effect is influenced by line load status and is unstable. .cndot.Combine with current controlled inverter .cndot.Not influenced by load status and is stable, primarily detect third harmonic component but inverter output voltage should be sine wave. generated by pole transformer. (3) Active countermeasure a. Frequency variation .cndot.Diverse inverter output frequency .cndot.System for modulating phase system when system power of feedback waveform of system voltage and supply losses. system for biasing frequency of oscillation circuit. Both are normally synchronized with frequency of line. b. Output voltage or current .cndot.Continuously and finely vary .cndot.Sine wave variation system and variation variation system inverter output voltage or current. system by quasirandom pattern. Effect may be lowered when a number of solar cell generations systems are parallelly operated. c. Output power variation system .cndot.System for initially imparting large (1) Active power variation .cndot.Continuously and finely vary inverter AC amplitude variation and system for output active power. imparting small variation and forming positive (2) Reactive power variation .cndot.Continuously and finely vary inverter feedback loop in reverse charge operation mode output reactive power. to amplify amplitude to detect resulting increase of voltage or frequency variation. In both systems, effect may be lowered when a number of solar cell generation systems are parallelly operated. 2. Countermeasure on line (1) Passive countermeasure a. Line zero-voltage .cndot.Check line zero-voltage after sub- Need to parallelly use transfer break. check device station breaker has been opened. (2) Active countermeasure a. Reactive power injection .cndot.Check line zero-voltage after substation .cndot.Very effective as post protection. system (Capacitor throw-in has been opened. If voltage is present, balance of system) reactive power is destroyed by connecting capacitor. __________________________________________________________________________ Note) There is shown an equilibrium range which can be detected by a conventional frequency and voltage monitor function at the time of equilibrium between power generation amount and load amount.
On the other hand, in a special high potential power transmission line, a "transfer break system" is used to avoid the reverse charge status. In the "transfer break", a break device is actuated by a break signal from a host substation to stop the system. However, in order to apply it to a low potential solar cell generation system, it is necessary to install a communication line to each of the solar cell generation systems, and it is not feasible because of a huge amount of cost.
A telephone line may be used as the communication line for the transfer break. In this case, a cost to install the communication line for each system may be significantly reduced. However, as described above, when a number of solar cell generation systems are widely used, the reverse charge phenomenon may possibly occur over a wide area. In such a case, all or most of the solar cell generation systems in the area of the reverse charge phenomenon must be broken.
In order to simultaneously break several tens to several thousands solar cell generation systems, a telephone line system which assigns the number to each of the users is not appropriate. To break one hundred systems, for example, telephone calls must be made at least one hundred times. Assuming that one user can be accessed in five seconds, it takes 500 seconds or more than eight minutes.